bigger disk brakes

As far as A body brakes are concerned, yours are about as good as it got, although they have some drawbacks today (small bolt pattern, parts availability). In 1973 Mopar changed the disk brake cars to the larger wheel bolt pattern (5 on 4.5" vs. 5 on 4"), but kept 11" rotors. This was done as a cost cutting measure allowing greater parts interchangeability among platforms. Late 70's B and R bodies used larger 11.75" rotors which can be fitted to the 73-up spindles using the appropriate caliper brackets.

A good place to start is here: http://www.moparaction.com/Tech/archive/disc-main.html

If you need even bigger brakes, of want to keep your small bolt pattern, you are stuck using aftermarket stuff.